This invention generally relates to a process for the treatment and disposal of waste including but not limited to sewage sludge (waste solids). The invention particularly relates to a method and apparatus for reliably and economically converting waste into biosolids, including Class A biosolids.
Waste and sewage sludge includes organic matter, moisture and nutrients that can be beneficial when returned to agricultural land. However, in its untreated form, waste and sewage sludge is unacceptable for land disposal. Untreated sewage sludge contains pathogenic organisms that are harmful to animals and man.
Known methods of processing sewage sludge involve thermal conditioning in such a manner as to improve the dewaterability of the sludge so that the solid and liquid phases are readily separated, as by sedimentation, filtration, and centrifugation. The solid and liquid phases are then disposed of or subjected to further treatment separately (Teletzke et al. U.S. Pat. No. 3,256,179; Teletzke et al. U.S. Pat. No. 3,697,417; Erickson et al. U.S. Pat. No. 3,824,186). Other methods have been used to obtain heat by the biological decomposition of organic substances (Moillet, U.S. Pat. No. 4,493,770), and to treat sludge by sterilization (Teletzke, U.S. Pat. No. 3,959,125).
Various methods and systems have been proposed to recycle sewage sludge to obtain useful materials, heat energy, or methane gas. Usually, the methods utilize an apparatus which includes an aeration container having a bottom wall to which an aeration apparatus is connected which introduces fresh air to sludge or biological refuse.
Sludge is introduced into an aerobic thermophilic pre-treatment digester where the waste solids are disinfected and preconditioned. The heat recovery portion of these systems may employ mechanical mixers in complex hot solids waste chambers that are disposed inside larger vessels. These heat recovery systems are very complex and, hence, expensive to manufacture and operate. The mechanical mixers have the further drawback of becoming clogged overtime. As the mixers become clogged or as sludge scale coats the walls of the chambers, it becomes necessary to disassemble and clean the chambers or vessels to restore adequate heat transfer efficiency. Disassembly shuts down the whole system.
Another problem with known systems results from the use of mechanical stirrers located within pre-treatment digesters. These mechanical stirrers also become clogged over time and require costly expensive maintenance. The pre-treatment digesters also have the drawback of requiring the costly introduction of air. Another shortcoming of the introduction of air into the pre-treatment digesters is that noxious odors are released from the pre-treatment digesters due to the introduction and contamination of air introduced therein.
Yet another shortcoming of known systems includes the use of costly water jackets that surround the pre-treatment digesters. These water jackets are complex and increase the cost of the pre-treatment digesters substantially. Further, maintenance of the water jackets is made difficult by the fact that they are usually permanently connected to the pre-treatment digesters to achieve efficient heat transfer.
Thus, there exists a need to solve the problems in the art that are articulated above. It is an object of the invention to solve these problems.